Determining the composition of an exoplanet’s atmosphere is a tricky endeavor. Water vapor has been detected in the atmospheres of a few exoplanets using two techniques: examining how light is scattered in a planet’s atmosphere when it passes in front of its host star, and imaging a planet that is far enough from its host star. Unfortunately, these techniques are not applicable to many exoplanets.
A new study led by California Institute of Technology graduate student Alexandra Lockwood has employed a novel technique to detect water vapor in the atmosphere of a “hot Jupiter”, a gas giant orbiting extremely close to its parent star. The hot Jupiter in question is tau Boötis b; previous research has identified carbon monoxide in its atmosphere.
According to a Caltech press release, Lockwood and colleagues adapted a technique known as radial velocity (RV) detection. The RV method ascertains the motion of a star attributable to the gravitational pull of an orbiting planet. In this scenario, the star’s motion is opposite the orbital motion of the planet, and the star’s light shifts in wavelength. The larger or closer a planet is to its parent star, the more pronounced the shift.
Although the RV method traditionally utilizes the visible spectrum, Lockwood and colleagues adapted it to infrared light to reveal tau Boötis b’s orbit and analyzed the wavelength shifts with spectroscopy to determine exactly which wavelengths of light are being emitted. The team pored over spectroscopic data on tau Boötis b obtained by the Near Infrared Echelle Spectrograph at the W. M. Keck Observatory in Hawaii. Because different molecular compounds emit different wavelengths of light, these data could shed further light on the planet’s atmospheric composition.
The team’s analysis showed that the atmosphere of tau Boötis b does contain water vapor. Their new technique has the added benefit of calculating the planet’s actual mass, rather than an estimated minimum mass as in prior studies. In order to determine the planet’s atmospheric makeup, Lockwood and colleagues had to first ascertain the relative motions of the planet and its star. This information, combined with the mass of the star, allow the mass the planet to be calculated.
For now, the modified RV method is applicable to only hot Jupiters. However, the team will continue to analyze the data on tau Boötis b and hope to widen their research to include cooler planets and dimmer stars as telescopes and spectrographs advance. The new research has been published in the February 24 online issue of The Astrophysical Journal Letters.